1,223 research outputs found
Spectropolarimetric multi line analysis of stellar magnetic fields
In this paper we study the feasibility of inferring the magnetic field from
polarized multi-line spectra using two methods: The pseudo line approach and
The PCA-ZDI approach. We use multi-line techniques, meaning that all the lines
of a stellar spectrum contribute to obtain a polarization signature. The use of
multiple lines dramatically increases the signal to noise ratio of these
polarizations signatures. Using one technique, the pseudo-line approach, we
construct the pseudo-line as the mean profile of all the individual lines. The
other technique, the PCA-ZDI approach proposed recently by Semel et al. (2006)
for the detection of polarized signals, combines Principle Components Analysis
(PCA) and the Zeeman Do ppler Imaging technique (ZDI). This new method has a
main advantage: the polarized signature is extracted using cross correlations
between the stellar spectra nd functions containing the polarization properties
of each line. These functions are the principal components of a database of
synthetic spectra. The synthesis of the spectra of the database are obtained
using the radiative transfer equations in LTE. The profiles built with the
PCA-ZDI technique are denominated Multi-Zeeman-Signatures. The construction of
the pseudo line as well as the Multi-Zeeman-Signatures is a powerful tool in
the study of stellar and solar magnetic fields. The information of the physical
parameters that governs the line formation is contained in the final polarized
profiles. In particular, using inversion codes, we have shown that the magnetic
field vector can be properly inferred with both approaches despite the magnetic
field regime.Comment: Accepted for publication in Astronomy and Astrophysic
PCA detection and denoising of Zeeman signatures in stellar polarised spectra
Our main objective is to develop a denoising strategy to increase the signal
to noise ratio of individual spectral lines of stellar spectropolarimetric
observations.
We use a multivariate statistics technique called Principal Component
Analysis. The cross-product matrix of the observations is diagonalized to
obtain the eigenvectors in which the original observations can be developed.
This basis is such that the first eigenvectors contain the greatest variance.
Assuming that the noise is uncorrelated a denoising is possible by
reconstructing the data with a truncated basis. We propose a method to identify
the number of eigenvectors for an efficient noise filtering.
Numerical simulations are used to demonstrate that an important increase of
the signal to noise ratio per spectral line is possible using PCA denoising
techniques. It can be also applied for detection of magnetic fields in stellar
atmospheres. We analyze the relation between PCA and commonly used well-known
techniques like line addition and least-squares deconvolution. Moreover, PCA is
very robust and easy to compute.Comment: accepted to be published in A&
First polarimetric observations and modeling of the FeH F^4 Delta-X^4 Delta system
Lines of diatomic molecules are more temperature and pressure sensitive than
atomic lines, which makes them ideal tools for studying cool stellar
atmospheres an internal structure of sunspots and starspots. The FeH F^4
Delta-X^4 Delta system represents such an example that exhibits in addition a
large magnetic field sensitivity. The current theoretical descriptions of these
transitions including the molecular constants involved are only based on
intensity measurements because polarimetric observations have not been
available so far, which limits their diagnostic value. We present for the first
time spectropolarimetric observations of the FeH F^4 Delta-X^4 Delta system
measured in sunspots to investigate their diagnostic capabilities for probing
solar and stellar magnetic fields. We investigate whether the current
theoretical model of FeH can reproduce the observed Stokes profiles including
their magnetic properties. The polarimetric observations are compared with
synthetic Stokes profiles modeled with radiative transfer calculations. This
allows us to infer the temperature and the magnetic field strength of the
observed sunspots. We find that the current theory successfully reproduces the
magnetic properties of a large number of lines in the FeH F^4 Delta-X^4 Delta
system. In a few cases the observations indicate a larger Zeeman splitting than
predicted by the theory. There, our observations have provided additional
constraints, which allowed us to determine empirical molecular constants. The
FeH F^4 Delta-X^4 Delta system is found to be a very sensitive magnetic
diagnostic tool. Polarimetric data of these lines provide us with more direct
information to study the coolest parts of astrophysical objects.Comment: 4 pages, 3 figure
Multiline Zeeman Signatures Through Line Addition
In order to get a significant Zeeman signature in the polarised spectra of a
magnetic star, we usually 'add' the contributions of numerous spectral lines;
the ultimate goal is to recover the spectropolarimetric prints of the magnetic
field in these line additions. Here we want to clarify the meaning of these
techniques of line addition; in particular, we try to interpret the meaning of
the 'pseudo-line' formed during this process and to find out why and how its
Zeeman signature is still meaningful. We create a synthetic case of line
addition and apply well tested standard solar methods routinely used in the
research on magnetism in our nearest star. The results are convincing and the
Zeeman signatures well detected; Solar methods are found to be quite efficient
also for stellar observations. We statistically compare line addition with
least-squares deconvolution and demonstrate that they both give very similar
results as a consequence of the special statistical properties of the weights.
The Zeeman signatures are unequivocally detected in this multiline approach. We
may anticipate the outcome that magnetic field detection is reliable well
beyond the weak-field approximation. Linear polarisation in the spectra of
solar type stars can be detected when the spectral resolution is sufficiently
high.Comment: 8 pages, accepted for publication in A&
Field-induced local moments around nonmagnetic impurities in metallic cuprates
We consider a defect in a strongly correlated host metal and discuss, within
a slave boson mean field formalism for the model, the formation of an
induced paramagnetic moment which is extended over nearby sites. We study in
particular an impurity in a metallic band, suitable for modelling the optimally
doped cuprates, in a regime where the impurity moment is paramagnetic. The form
of the local susceptibility as a function of temperature and doping is found to
agree well with recent NMR experiments, without including screening processes
leading to the Kondo effect.Comment: 7 pages, submitted to Phys Rev
A near-IR line of Mn I as a diagnostic tool of the average magnetic energy in the solar photosphere
We report on spectropolarimetric observations of a near-IR line of Mn I
located at 15262.702 A whose intensity and polarization profiles are very
sensitive to the presence of hyperfine structure. A theoretical investigation
of the magnetic sensitivity of this line to the magnetic field uncovers several
interesting properties. The most important one is that the presence of strong
Paschen-Back perturbations due to the hyperfine structure produces an intensity
line profile whose shape changes according to the absolute value of the
magnetic field strength. A line ratio technique is developed from the intrinsic
variations of the line profile. This line ratio technique is applied to
spectropolarimetric observations of the quiet solar photosphere in order to
explore the probability distribution function of the magnetic field strength.
Particular attention is given to the quietest area of the observed field of
view, which was encircled by an enhanced network region. A detailed theoretical
investigation shows that the inferred distribution yields information on the
average magnetic field strength and the spatial scale at which the magnetic
field is organized. A first estimation gives ~250 G for the mean field strength
and a tentative value of ~0.45" for the spatial scale at which the observed
magnetic field is horizontally organized.Comment: 42 pages, 17 figures, accepted for publication in the Astrophysical
Journal. Figures 1 and 9 are in JPG forma
Magnetic fields and differential rotation on the pre-main sequence I: The early-G star HD 141943 - brightness and magnetic topologies
Spectroscopic and spectropolarimetric observations of the pre-main sequence
early-G star HD 141943 were obtained at four observing epochs (in 2006, 2007,
2009 and 2010). The observations were undertaken at the 3.9-m Anglo-Australian
Telescope using the UCLES echelle spectrograph and the SEMPOL
spectropolarimeter visitor instrument. Brightness and surface magnetic field
topologies were reconstructed for the star using the technique of least-squares
deconvolution to increase the signal-to-noise of the data.
The reconstructed brightness maps show that HD 141943 had a weak polar spot
and a significant amount of low latitude features, with little change in the
latitude distribution of the spots over the 4 years of observations. The
surface magnetic field was reconstructed at three of the epochs from a high
order (l <= 30) spherical harmonic expansion of the spectropolarimetric
observations. The reconstructed magnetic topologies show that in 2007 and 2010
the surface magnetic field was reasonably balanced between poloidal and
toroidal components. However we find tentative evidence of a change in the
poloidal/toroidal ratio in 2009 with the poloidal component becoming more
dominant. At all epochs the radial magnetic field is predominantly
non-axisymmetric while the azimuthal field is predominantly axisymmetric with a
ring of positive azimuthal field around the pole similar to that seen on other
active stars.Comment: 18 pages, 17 figures, accepted by MNRA
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